In a known machine vice with force amplifier of this kind (DE No. A-3 438 900), the fixed jaw is releasably connected to the base member by means of screws. It can be screwed selectively to one of the two longitudinal end regions of the base member. If it is screwed onto the end of the base member facing away from the reaction plate, external gripping of workpieces is possible. For internal gripping of workpieces the fixed jaw is released from the one end of the base member and screwed onto the other end of the base member, at which the reaction plate is also fixed. The movable jaw is for its part also connected by means of screws to a slide guided in the base member, this slide also having the projection with the nut. In order to fix the fixed jaw relative to the base member and the movable jaw relative to the slide, groove-key connections are also provided between the associated parts. Although the producer of this known machine vice recommends that the screws serving for fixing the fixed jaw to the base member and the movable jaw to the slide should be initially tightened with a small torque and then the two jaws should be forced together with maximum tightening and all fixing screws should be tightened up with a higher torque in this pretensioned position, it has been ascertained that the jaws yield by some hundredth of a millimeter in the tightening direction of the machine vice after releasing the screw spindle and renewed tightening of the same under high pressure. Such a yielding is however especially disadvantageous when the machine vice is fitted on NC machine tools, since an alteration of the position of the work piece relative to the base member of around a hundredth of a millimeter arises from the yielding of the jaws. The so called positional accuracy therefore suffers from the yielding of the jaws, especially of the fixed jaw. Furthermore, the production costs in the known machine vice are increased in that the fixed jaw and base member as well as the movable jaw and carriage are each formed in two parts and have to be provided with additional groove-key connections for the accurate mutual fixing of these parts.
There are moreover already known machine vices with force amplifier (DE-A No. 1 933 733), in which the fixed jaw is formed unitarily with the base member and the movable jaw unitarily with the projection. With this machine vice however only an outer gripping of work pieces is possible since the force amplifier can fundamentally act only in one direction.
The invention is based on the problem of providing a machine vice with a force amplifier of the initially mentioned kind which has a particularly highest stability and accordingly also a high positional accuracy, which is simple in construction and which nevertheless makes possible the selective outside gripping and inside gripping.
This is obtained according to the invention in that the fixed jaw is formed unitary with the base member and the movable jaw is formed unitary with the projection and in that the base member has at its one end a second fixing surface for the reaction plate and corresponding threaded bores for its screws, so that the reaction plate with the tightening device abutting thereon can be fixed selectively on one of the ends of the base member.
Through the unitary formation of the fixed jaw with the base member on the one hand and of the movable jaw with the projection on the other hand there arises an extremely high stability of the machine vice. Hereby it is obtained that, with tightening under high pressure the fixed jaw in particular hardly yields at all in the tightening direction, so that the desired high positional accuracy is guaranteed. Moreover the machine vice comprises fewer individual parts and is accordingly cheaper in production. Nevertheless both an outside gripping of workpieces and also an inside gripping of workpieces, such as e.g. hollow bodies, are possible. In the former case the reaction plate is screwed onto the fixing surface of the end of the base member directed away from the fixed jaw. For inside gripping the reaction plate is released from this fixing surface and then the screw spindle is completely screwed out of the spindle nut. The reaction plate can then be screwed, with the whole tensioning device, onto the fixing surface which is at the same end of the base member on which also the fixed jaw is arranged. In both cases a tightening under high pressure is possible, since both in outside gripping and also in inside gripping the moving jaw is moved in each case away from the reaction plate.
Advantageous embodiments of the invention are characterised in the sub claims.